MAPREDUCE-4991. coverage for gridmix (Aleksey Gorshkov via tgraves)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1463806 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HADOOP-9016-trunk-gd